MLECO-4825: Adding Arm Corstone-315 support

This patch adds initial support for MPS4 based Arm Corstone-315 FVPs.
The applications will execute on the FVP but with INITSVTOR set to
`0x12000000` explicitly. The default value is the 64kiB code region
at `0x11000000`. The linker scripts will be changed for the initial
boot logic and vector table to be moved to the code region.

This patch adds `source/hal/source/platform/mps4` directory. There is
considerable overlap with MPS3 platform and this is expected to be the
case until the support for MPS4 matures. Refactoring to pull in
common bits from these targets will follow. Same goes for the CMake
build support added under `scripts/cmake/platforms/mps4`.

Change-Id: I981be9e1ec57cfedcf7d340b4f19e5eb40b5cbd3
Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>
Tested-by: mlecosys <mlecosys@arm.com>
Reviewed-by: Conor Kennedy <conor.kennedy@arm.com>
Reviewed-by: Alex Tawse <alex.tawse@arm.com>
Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>